Processes for preparing metal cation-promoted silver catalysts for the production of ethylene oxide are extensively described in the patent literature. The vast majority of these processes employ impregnation techniques wherein solutions containing solubilized compounds of silver and metal cation promoters are used to impregnate a porous carrier or support followed by heat treatment of the impregnated support to effect deposition of the silver and metal cation on the support. Impregnation processes differ from processes for making coated catalysts employ techniques wherein silver and metal-containing promoters are coated onto a catalyst support from an emulsion or slurry followed by a heating step to remove the liquid present from the carrier and effect deposition of the silver and metal-containing promoter. Coated catalysts are generally considered today to be less satisfactory than impregnated catalysts in commercial practice because it is generally believed that coating of silver into the interior surfaces of the carrier and consequently, the coated catalysts are more susceptible to silver loss by mechanical abrasion.
The impregnation methods described in the art for preparing ethylene oxide catalysts include a wide variety of methods of depositing silver and metal cations onto a carrier. These methods are generally distinguished by the process conditions they employ such as low-temperature impregnation, high temperature impregnation, activation in an inert gas atmosphere and/or choice of solvent for the silver impregnating solution.
Impregnation processes are characterized by their employing either a coincidental (or simultaneous) method of depositing silver and metal-containing promoter onto the carrier or a sequential method of addition wherein silver is added either before or after the metal-containing promoter. The addition of silver to a carrier subsequent to the addition of metal-containing promoter is referred to herein as a "metal-first" sequential process of preparation, while the addition of silver to the carrier prior to the addition of the metal-containing promoter is referred to herein as a "silver-first" method of preparation. The coincidental (or simultaneous) addition of silver and metal-containing promoter to a carrier is referred to herein as a "coincidental method" or "coimpregnation method" of preparation. The use of the term "addition" of a metal-containing promoter and/or silver to a carrier is meant to include the steps of impregnating the porous carrier with a solution containing silver and/or metal-containing promoter or precursor thereof, as the case may be, followed by deposition of same upon the carrier, usually by heat treatment.
The comparative performance of catalysts produced by coincidental and sequential methods of impregnation has been reported in the art. Thus, U.S. Pat. No. 3,563,914 to Wattimena and Belgian Patent No. 793,658 and U.S. Pat. Nos. 3,962,136, 4,101,115 and 4,012,425 to Nielsen et al contain comparative data illustrating the relative inefficiency of alkali metal-containing catalysts produced by a silver-first sequential method of addition relative to a coincidental method of addition. U.S. Pat. No. 4,207,210 to Kilty, based upon British specification No. 1 489 335, describes an alkali metal-first process for preparing ethylene oxide catalysts which is said to provide catalysts equivalent or even superior to those produced by coincidental methods of deposition such as set forth in the aforementioned U.S. patents to Nielsen et al.
While other patents in the art directed to silver-first methods of preparing alkali metal-containing catalysts do not provide sufficient data to allow side-by-side comparisons to be made between the coincidental and sequential impregnation processes, nevertheless, the data which are provided appear to indicate that silver-first methods are often the less preferred methods. U.S. Pat. No. 4,033,903 to Maxwell, for example, discloses a silver-first method of addition wherein used ethylene oxide catalysts are reactivated by the addition of an alkali metal-containing promoter to the aged catalyst. The process of the patent is said to be equally effective for enhancing the efficiency of freshly prepared catalysts by employing a heat treatment step intermediate to the steps of silver addition and alkali metal addition to the carrier. The effectiveness of this method of preparation seems somewhat doubtful, however, in view of the data shown in Table III of the patent wherein catalyst R and T, catalysts prepared by a silver-first method are shown to be inferior to catalyst Q, a silver catalyst containing no alkali metal-containing promoter.
U.S. Pat. No. 4,066,575 to Winnick describes a process of catalyst preparation characterized by an activation step wherein the carrier is heated in an inert gas atmosphere following its impregnation with a silver solution. An alkali metal-containing promoter is thereafter deposited on the carrier employing as a solvent for the alkali metal, water or a lower alkanol such as, methanol, ethanol or propanol. Great Britain patent application No. 2,045,636A attempts to distinguish itself from the prior art processes by its low-temperature deposition technique whereby the carrier impregnated with a silver-containing solution is maintained at temperatures below 200.degree. C. prior to the so-called post deposition of alkali metal. The suggested solvents for such post-deposition of alkali metal are water and ethanol. German Offenlegungsschrift No. 2,914,640 discloses a sequential order of impregnation wherein silver is initially applied to the carrier from a suspension and the carrier thereafter immediately dried. Alkali metal is then added to the carrier from a solution using water as the solvent. U.S. Pat. No. 4,248,740 to Mitsuhata et al describes a catalyst preparation procedure employing a silver-first order of addition. The patentees recommend impregnating the carrier with an alkali metal solution containing water or a lower alcohol, such as methanol, ethanol or propanol. The solvent is then catalyst to above 200.degree. C., a critical feature of the described evaporated, care being taken to prevent heating of the process. In U.S. Pat. No. 4,168,247 to Hayden et al, there is described a preparation procedure for catalysts identified by the numbers 34-37 which consists of a silver-first order of addition. The alkali metal-containing promoters were dissolved in water with further addition of methanol, and the resulting solution used to impregnate the carrier.
Japanese patent application No. 142,421/78 (Kokai No. 79,193/79) discloses a "post-treatment" of a used or stabilized silver catalyst by impregnating such catalyst with a solution containing an alkali metal-containing promoter, an organic compound capable of forming a complex salt with silver ion and an alcohol of 1 to 4 carbon atoms. No alcohol other than methanol was used in the impregnating solution described in the examples.
U.S. Pat. No. 4,419,276 to Bhasin et al and U.S. Pat. No. 4,455,392 to Warner et al disclose sequentially preparing ethylene oxide production catalysts by impregnating a carrier with a silver salt and then with a compound of at least one metal-containing promoter. These patents involve the use of certain solvents in at least one of the impregnating solutions. These patents further disclose that the particular metal promoter employed is not critical and may include the one or more alkali metals; one or more alkaline earth metals; or one or more other promoters, such as thallium, gold, tin, antimony, rare earths and the like. The catalysts produced are said to be equally as efficient as catalysts produced by coincidental methods of preparation.
Supported, silver-containing, alkylene oxide catalysts often include one or more metal-containing promoters other than alkali metals or alkaline earth metals to provide enhanced performance, e.g., enhanced activity and/or efficiency and/or stability. It would be advantageous to provide processes to prepare such catalysts, e.g., so as to provide enhanced catalyst preparation processes and/or catalysts and/or alkylene oxide processes which make effective use of such promoters.
In characterizing this invention, the terms "conversion", "selectivity", and "yield" are employed as defined in U.S. Pat. No. 3,420,784, patented Jan. 7, 1969, at column 3, lines 24-35 inclusive. This definition of "selectivity" is consistent with that disclosed in U.S. Pat. No. 2,766,261 at column 6, lines 5-22, and U.S. Pat. No. 3,144,916, lines 58-61. The definitions of "yield" and "conversion" have more varied meaning in the art and are not to be employed as defined, for example, in the aforementioned U.S. Pat. No. 2,766,261. The terms "efficiency" and "selectivity", as used throughout the specification and claims are intended to be synonymous.